[stack] Fix clear function with dtor

Small logic fix

src/rnd_stack.c

@@ -60,9 +60,8 @@ int rnd_stack_clear(struct rnd_stack *stack, int (*dtor)(void*))
  }
  p += stack->elem_size;
  }
- } else {
- stack->size = 0;
  }
+ stack->size = 0;
  return 0;
 }